Polymers have poor stability against heat and light and are, therefore, easily oxidized and degraded when exposed to a high-temperature environment or a strong light, so that the service life required for a plastic article cannot be attained. In order to prevent such oxidation and degradation, stabilizers such as a phenolic antioxidant, a phosphorus-based antioxidant, a sulfur-based antioxidant, a hydroxylamine compound, a hindered amine compound, an ultraviolet absorber and an acid scavenger are commonly added. In particular, phenolic antioxidants have high stabilization effect against thermal oxidation of a polymer obtained from a monomer having an ethylenically unsaturated bond, such as polyolefin, and are capable of imparting the polymer with resistance against oxidation and discoloration during storage; therefore, phenolic antioxidants have high utility value as a stabilizer of a polymer obtained from a monomer having an ethylenically unsaturated bond, such as polyolefin.
Conventionally, in the stabilization of a polymer obtained from a monomer having an ethylenically unsaturated bond such as polyolefin, a long-term stabilization is attained by blending a variety of stabilizers into a fine powder-form polymer obtained by using a Ziegler catalyst and/or a metallocene catalyst and then molding the resulting polymer using a granulator or the like into the form of granules that are easy to handle.
However, because of the recent advance of the polymerization technologies, it is now possible to directly obtain a polymer in the form of granules. Since the blending of a stabilizer for stabilization of a polymer and the subsequent molding of the polymer into the form of granules using a granulator expose the polymer to a high-temperature environment in the melt-kneading step thereof, deterioration of the physical properties of the polymer, such as a reduction in the molecular weight, is inevitable. In addition, the need of adding an excessive amount of stabilizer to mitigate the poor dispersion thereof is economically disadvantageous.
In view of this, in order to omit the step of blending a stabilizer by melt-kneading of a polymer, methods in which a stabilizer is added before or during the polymerization of a monomer have been researched and developed; however, there is still a problem that phenolic antioxidants have an effect of inhibiting the catalytic activities of polymerization catalysts.
For example, in Patent Document 1, it is described that phenolic antioxidants that are widely used in polyolefins, such as tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxymethyl)methane, inhibit the catalytic activities of polymerization catalysts and that such phenolic antioxidants, therefore, cannot be added before polymerization. In Patent Document 1, a method in which a complex is formed by an ether compound on a Ziegler catalyst supported on magnesium chloride is proposed.
In addition, in Patent Documents 2 to 4, the present inventors have proposed methods of stabilizing a polymer without impairing the activity of a polymerization catalyst, which comprise the step of masking a phenolic antioxidant by mixing it with an organic aluminum compound usually used in olefin polymerization in an existing catalyst feed tank or polymerization vessel.